the embryonic human tail

the embryonic human tail - the embryonic human tail Humans...

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the embryonic human tail Humans are classified by taxonomists as apes; one of the defining derived characters of apes is the lack of an external tail. However, human embryos initially develop tails in development. At between four and five weeks of age, the normal human embryo has 10-12 developing tail vertebrae which extend beyond the anus and legs, accounting for more than 10% of the length of the embryo (Fallon and Simandl 1978 ; Moore and Persaud 1998 , pp. 91-100; Nievelstein et al . 1993 ). The embryonic tail is composed of several complex tissues besides the developing vertebrae, including a secondary neural tube (spinal cord), a notochord, mesenchyme, and tail gut. By the eighth week of gestation, the sixth to twelfth vertebrae have disappeared via cell death, and the fifth and fourth tail vertebrae are still being reduced. Likewise, the associated tail tissues also undergo cell death and regress. Figure 2.4.3. A histological cross-section of the human embryo's tail at Carnegie stage 14 . The complex structures present in the human tail were visualized with light microscopy in this image. At Carnegie stage 14 (about 32 days old), the human tail is composed of neural tube ( n , doubled in a large fraction of embryos), notochord ( c ), developing vertebrae (somites, s ), gut ( g ), and mesenchyme ( m ). These specialized structures extend for the length of the tail, and the cells of all these structures die and are digested by immune system macrophages within the next two weeks of embryonic development. (Modified from plate 22 of Fallon and Simandl 1978 ) Using light and scanning electron microscopy, several detailed analyses of the embryonic human tail have shown that the dead and degenerating tail cells are ingested and digested by
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macrophages (macrophages are large white blood cells of the immune system which more normally ingest and destroy invading pathogens such as bacteria) (Fallon and Simandl 1978 ; Nievelstein et al . 1993 ; Sapunar et al . 2001 ; Saraga-Babic et al . 1994 ; Saraga-Babic et al . 2002 ). In adult humans, the tail is finally reduced to a small bone composed of just four fused vertebrae (the coccyx) which do not protrude from the back (Fallon and Simandl 1978 ; Sapunar et al . 2001 ) (see Figure 2.4.1). The regression of the human embryonic tail can be clearly seen in the fantastic images available at the Multi-dimensional Human Embryo site, where online images of three-dimensional MRI scans of live human embryos are archived. Different levels of maturity of the human embryo are
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the embryonic human tail - the embryonic human tail Humans...

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